1. Field of the Invention
The present invention relates to a surge current detection device provided in a protector, which guards and protects a piece of equipment to be protected, such as a communication circuit unit, an electric power supply circuit unit and so on, from a surge current caused by a thunderbolt or the like and penetrated into conductors such as communication lines, power supply lines and so on, which are connected to the piece of equipment to be protected, and more particularly relates to such a surge current detection device for detecting a penetration state of the surge current by utilizing a magnetic field, which is generated around the conductors due to the penetration of the surge current thereinto.
2. Description of the Related Art
Recently, while a quality of a piece of equipment to be protected, such as a communication circuit unit, an electric power supply unit and so on are being improved, the number of damage cases, in which pieces of equipment to be protected are damaged by the thunderbolts or the like, are tending upward. For example, when a thunderbolt strikes a peripheral facility of a building, a surge current flows around a location at which the thunderbolt strikes. When the surge current penetrates into an interior of the peripheral facility, a piece of equipment to be protected may be damaged. Thus, a surge current detection device for detecting the penetration of the surge current is needed.
In order that the piece of equipment to be protected is not subjected to damage, a surge protection device (SPD), which may be referred to as a protector, is provided in the facility. However, even in the protector having a function of protecting the piece of equipment to be protected from the thunderbolt surge current, protection elements (i.e., thunderbolt protection elements) such as an arrester, a varistor and so on provided in the protector may be damaged. Thus, similar to the facility to be protected, a surge current detection device for detecting the penetration of the surge current is needed for the protectors.
In the protector, among others, the interior protection elements such as the varistor, the arrester and so on are inevitably subjected to a gradual deterioration due to applications of a surge current and a surge voltage thereto. Since the respective inherent functions of the deteriorated elements decline, it is important to prematurely exchange the deteriorated protector for a fresh one.
As prior arts for resolving the problems as mentioned above, prior art surge current detection devices are disclosed in, for example, the following Patent Documents 1 to 3:    [Patent Document 1]: JP-2005-150657 A    [Patent Document 2]: JP-2006-244889 A    [Patent Document 3]: JP-2007-242569 A
In the surge current detection device disclosed in Patent Document 1, a degree of deterioration of a protection element such as a varistor is determined by changing a color of a heat sensitive material, using Joule heat caused by a surge current flowing through a conductor on a thunderbolt. In particular, the heat sensitive material is placed on the protection element such as the varistor, and the Joule heat is generated when the surge current flows through the protection element and when the protection element is operated, to thereby vary the color of the heat sensitive material.
In surge current detection devices disclosed in Patent Documents 2 and 3, similar to the aforesaid case, a determination method uses Joule heat, and a degree of deterioration of a protection element is determined by using a shrinkage of a metal composed of a heat-shrinkable material. For example, when the shrinkable metal material is shrunken due to the Joule heat of the surge current, the deterioration determination is carried out by using a mechanical mechanism which is made to expose the hidden degradation mark and can be viewed according to the shrinkage power.
Also, a technique of a magnetic displaying medium, which can visually confirms information contents of a card, and which, however, does not belong to the technical field of the protector concerned, is disclosed in the following Patent Document 4:    [Patent Document 4]: JP-H05-016578 A
The magnetic displaying medium disclosed in Patent Document 4 includes a substrate having an information storage section and a magnetic display section provided thereon. The information storage section is formed of either a magnetic tape or an IC (integrated circuit) memory. The magnetic display section includes a substrate and a recording layer provided thereon. The recording layer is formed by directly coating the substrate with a plurality of microcapsules or by coating the substrate with the plurality of microcapsules through the intermediary of a suitable intermediate layer. Each of the microcapsules contains a suitable liquid, and a plurality of magnetic particles suspended in the liquid and easily affected by a magnetic field. The magnetic display section is constituted so that a piece of information can be visually displayed on the recording layer in accordance with information stored on the information storage section, and so that the displaced piece of information can be erased from the recording layer, if necessary.
Further, a technique concerning such microcapsules is disclosed in the following Patent Document 5:    [Patent Document 5]: JP-H11-076801 A
Nevertheless, the prior art surge current detection devices, as disclosed in Patent Documents 1 to 3, have the following problems (a), (b) and (c) to be solved as mentioned hereinbelow:
(a) The prior art surge current detection devices utilize the Joule heat of the surge current flowing through the conductor in order to determine a degree of deterioration of the protection element such as the varistor, the arrester and so on. Since the surge current must quickly flow through the conductor, it is necessary to make a resistance of the conductor very small. For this reason, the surge current flows through the conductor for a very short period of time, and thus an amount of Joule heat is small, resulting in decline in a sensibility of deterioration detection.
(b) Since the prior art surge current detection devices utilizing the shrinkable metal material include the physical mechanism in which a degree of shrinkage of the shrinkable metal material is amplified so as to be able to be visually displayed, it is difficult to downsize these devices.
(c) In the prior art surge current detection devices utilizing the heat sensitive material and the shrinkable metal material, it is difficult or impossible to reuse these materials, and thus the used materials are put into the discard together with the protectors, to thereby waste the many resources.
In short, in the prior art surge current detection devices, there are the problems that the sensibility of deterioration detection is inferior, that it is difficult to downsize the devices, and that it is impossible to reuse the devices.
In order to solve these problems, for example, although it is considered that the recording layer composed of the microcapsules, as disclosed in Patent Documents 4 and 5, is applied to the prior art surge current detection devices to thereby detect the surge current, it is very difficult to increase the sensibility of deterioration detection because the surge current caused by the thunderbolt flows through the conductor for an extremely short period of time.